I’m enjoying reading An Odyssey: A Father, A Son and an Epic. The author, Daniel Mendelsohn, briefly introduced where the word mentor came from.
In The Odyssey, Mentor is the name of a friend of Odysseus. The goddess Athena disguises herself as Mentor to provide counsel to Odysseus’ son, Telemachus. Mentor was someone we assumed Telemachus trusts and would listen to.
And this is the genesis of mentor. It becomes a noun, meaning an experienced and trusted adviser.
In discussing the word's origins, Wikipedia offers more.
... a mentor is someone who imparts wisdom to, and shares knowledge with, a less experienced colleague.
The key words are: experienced, trusted adviser, wisdom, shares. To this we need to add the words relevant and willing; otherwise, the mentor becomes just another person offering advice.
A mentor is an experienced, trusted adviser who shares relevant wisdom with a willing individual.